Sodium thiosulfate supersaturated solution

Supersaturation.—One of the most frequent difficulties in crystallization is due to super saturation. This condition arises when the normal saturation concentration of a salt solution is exceeded without the appearance of any crystals, and as the solution cools further it becomes steadily more supersaturated. When crystallization finally starts, it proceeds with great rapidity, forming a mass of poorly defined crystals unsuited to drying. The tendency toward supersaturation is most marked in the case of very soluble substances which form viscous or syrupy solutions. Lead acetate, sodium thiosulfate, ferric nitrate, and sulfuric acid are good examples. 

The first solution, made at room temperature, was an unsaturated solution. That means it held less solute than it was really capable of holding. Therefore, when you added the extra crystal, it dissolved. The second solution, with heat applied, was a saturated solution. This means that it held all the solute it is normally capable of holding, plus a little excess that didn t dissolve. Therefore, when you later added an extra crystal, it didn t dissolve, but fell to the bottom with the other excess undissolved crystals. The third solution, made at boiling temperature, was a supersaturated solution. The boiling forced it to absorb much more solute than it would normally hold. The extra crystal that you later added seemed to be the center of an area where new crystals of sodium thiosulfate grew. Of course, they were not really new. They separated out from the solution as it cooled, and they would have done so even if you hadn t added another crystal. The one you added merely served as a seed crystal, or starter for the others to cluster around. The crystal growth continued until the solution was only saturated rather than supersaturated. 

Monodispersed colloids can also be obtained by the La Mer method [ 11,12] in which light supersaturation in the system is maintained over a long period of time thanks to a slow chemical reaction. One example of such a process is the reaction between a dilute sodium thiosulfate solution and dilute sulfuric or hydrochloric acid, i.e. 

Sometimes it is possible to obtain a supersaturated solution, a solution that contains more dissolved substance than a saturated solution does. For example, the solubility of sodium thiosulfate, Na2S203, in water at 100°C is 231 g/lOOmL. But at room temperature, the solubility is much less—about 50 g/100 mL. Suppose you prepare a solution saturated with sodium thiosulfate at 100°C. You might expect that as the water solution is cooled, sodium thiosulfate would crystallize out. In fact, if the solution is slowly cooled to room temperature, this does not occur. Instead the result is a solution in which 231 g of sodium thiosulfate is dissolved in 100 mL of cold water, compared with the 50 g you would normally expect to find dissolved. 

Supersaturated solutions are not in equilibrium with the solid substance. If a small crystal of sodium thiosulfate is added to a supersaturated solution, the excess immediately crystallizes out. Crystallization from a supmaaturated solution is usually quite fast and dramatic (see Figure 12.4). 

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